Also, before using iso-propyl alcohol to clean off the adhesive, first use you fingernail to carefully pull up the edge of a strip and pull on it to remove large sections easily. The bottom strip is fiddly to get an edge of but should peel off in a single piece if you get hold of either end. Makes final cleanup with iso quick and easy.

The key is to be patient. Let the adhesive slowly release. Worked perfectly. I find that old credit card, ID cards, etc work in place of an iFixit plastic card. I strongly recommend the iMac Opening Tool though.

I can't, for the life of me, get the left screw out of the cowling. Had to pull both out together. Now having a very hard time getting the cowling back in to replace the fan.

Yes. Initialise the new drive as GUID, MacOS Extended, Journaled.

Then install and all should go well.